Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Electrochemical conversion of energy has proven to be an important alternative propulsion source for automotive applications. In a fuel cell system, energy is produced through the cold combustion (reaction) of hydrogen and oxygen. The reaction takes place in a fuel cell stack, in which individual fuel cells are stacked together in series to generate increasingly larger quantities of electricity.
While they are a promising development in automotive technology, fuel cells are characterized by a high operating temperature which presents a significant design challenge from the standpoint of maintaining the structural and operational integrity of the fuel cell stack. Maintaining the fuel cell stack within the temperature ranges that are required for optimum fuel cell operation depends on a highly-efficient cooling system which is suitable for the purpose. The heat generated by the energy conversion process carried out in a fuel cell stack is removed from the stack through a coolant circulation system.
A typical coolant circulation system for a fuel cell vehicle includes a pump which pumps coolant through a fuel cell stack, a chiller through which the coolant from the stack may flow for cooling, a bypass loop which bypasses the chiller, a two-way coolant flow valve which distributes the coolant through either the chiller or the bypass loop, and stack inlet and stack outlet temperature sensors. If the coolant temperature as measured by the stack inlet and stack outlet temperature sensors is excessively high, the coolant flow valve permits flow of the coolant through the chiller to cool the coolant to a desired temperature before the coolant is distributed into the stack. On the other hand, if the temperature of the coolant is optimum or too low, the coolant flow valve shunts the coolant from the chiller through the bypass loop. The coolant remains at substantially the same temperature as it is distributed into the stack.
The two-way valve typically includes a performance-map thermostat having a wax expansion element. The expansion element is electrically coupled to a heating system which controls the temperature of the expansion element independently of the coolant temperature. The density and volume of the expansion element change depending on the temperature of the element. Therefore, when it is unheated, the expansion element is in a contracted configuration and blocks flow of coolant from the chiller to the stack while facilitating flow of the coolant from the bypass loop to the stack. When it is heated, on the other hand, the expansion element expands and facilitates flow of coolant from the chiller to the stack while blocking flow of the coolant from the bypass loop to the stack.
The heating system which induces heating of the expansion element is controlled by the stack inlet and stack outlet temperature sensors. Since heating of the expansion element acts through a linear (ohmic) resistance, the ohmic resistance of the expansion element is correlated with the valve outlet or stack inlet temperature of the coolant.
Therefore, by determining the stack inlet temperature of the coolant based on the ohmic resistance of the expansion element, the stack inlet temperature sensor can be eliminated from the coolant circuit. This reduces costs and eliminates sensor assembly and defect problems. Furthermore, additional space is made available for other components or systems.